Electronic addressing or logic devices, for instance for storage or processing of data are at present synonymous with inorganic solid state technology, and particularly crystalline silicon devices. Even if such devices have shown themselves technically and commercially very successful, they are encumbered with a number of disadvantages. Particularly they are encumbered with a complex architecture which leads to high cost and a loss of data storage density. Within the large subgroup of volatile semiconductor memories based on inorganic semiconductor material, the circuitry must constantly be supplied with electric current with a resulting heating and high electric power consumption in order to maintain the stored information. Non-volatile semiconductor devices on the other hand avoid this problem with a resulting reduction in the data rate accompanied by high power consumption and large degree of complexity. A number of different architectures has been implemented for memory chips based on semiconductor material and reflects a tendency to specialization with regard to different tasks. Matrix addressing of memory locations in a plane is a simple and effective way of achieving a great number of accessible memory locations with a reasonable number of lines for electrical addressing. In a square grid with n lines in each direction the number of memory locations hence scales as n.sup.2. In one form or another this is the basic principle which at present is implemented in a number of solid state semiconductor memories. In these cases however, each memory location must have a dedicated electronic circuit which communicates to the outside via the grid intersection point as well as a volatile or non-volatile memory element, typically a charge storage unit.
There has in the art been proposed a number of devices which are meant to realize addressable passive memory elements based on the use of an organic memory medium. Thus there is from JP-A-4-145664 (Takeda, assigned to Canon Inc.) known an organic electronic element wherein a thin film is provided between overlying and underlying electrodes, the underlying electrode being provided on a suitable substrate and the overlying electrode crossing the underlying electrode perpendicularly. By varying the electrical voltage between the electrodes the conductivity of the organic thin film is affected. This conductivity may be maintained permanently and used for representing a memory state in the thin film between a pair of electrodes. There is, however, given no indication how this method and device may be used for addressing in large passive matrices. JP-A-62-95882-(A) (Yamamoto, assigned to Canon Inc.) discloses a memory cell with a first underlying electrode formed by depositing copper on a glass substrate and over the electrode depositing a thin film of a charge-transfer organometallic complex, in this case Cu--TCNQ, whereafter an overlying electrode formed by depositing an aluminium paste on the thin film. If the electric potential of the first electrode is higher than that of the second electrode, the thin film is kept in a high resistance state until the electric field reaches a threshold intensity and is thereafter switched to a low resistance state. No indication is here given that such memory elements straightforwardly may be provided in large passive matrices. Generally it is, however, well known to form a memory device wherein the memory medium is a bistable switchable thin film in the form of an organic complex compound of the charge-transfer type, cf. also JP-A-62-95883 with the same inventor and applicant, wherein transistor switches are used in each memory element for addressing.
In JP-A-3-137896 (Taomoto, assigned to Matsushita Giken K.K) there is once more proposed a memory element which uses an organic thin film which may be switche bistably between a high resistance state and a low resistance state by application of an electric field and maintains the momentary resistance state after the electric field has been withdrawn. Further this element may change the state very fast at high temperature, but slower at low temperature. Again an organic thin film is located between an overlying and an underlying electrode and provided on a substrate. It is stated that the switching takes place faster and faster with increasing temperature, but nothing is said of the use of a memory element of this kind in large passive matrices and whether it is suitable for passive matrix addressing. Further there is from JP-A-3-137894 (Asakawa, assigned to Matsushito Giken K.K.) known to provide thin films between overlying and underlying electrode matrices. In the actual case the matrix is shown as a 6.11 matrix, hence with a total of 66 elements. The thin film is a vapour deposited phtalocyanine film. If a voltage higher than a threshold value is applied to an electrode intersection, an on state is stored. When a voltage equal to the threshold value is applied, the intersection point is irradiated with light, such that the on state is stored in this part and information supplied in the form of light may be written directly into the matrix. When a reversed voltage is applied to the intersection point, the on-state is erased. Hence a structure which realizes a memory function both with an electric signal and a light signal is obtained. Even if there here are used a 6.11 matrix, it is in no way evident that this bistable switchable memory element will function without error when addressing in a passive matrix with a large number of memory elements.
Finally there is in a paper by Z.Y. Hua & G.R. Chen, "A new material for optical, electrical and electronic thin film memories", Vacuum 43, No. 11, pp. 1019-1023 (1992), described a new category of eraseable memory media which allows the realisation of memory elements which may be switched bistably by supplying energy in the form of heat, electrical fields or light radiation under different conditions. These memory media are based on the above-mentioned organometallic charge-transfer complex M(TCNQ) formed in 7,7,8,8-tetracynoquinodimetan (C.sub.12 H.sub.4 N.sub.4) which acts as an electron acceptor molecules with different metals (M) as electron-rich donors. Hua & Chen propose the use of M(TCNQ) in an electrically erasable memory by for instance forming a matrix of switching elements based on Cu(TCNQ) between a set of underlying electrodes, for instance of aluminium, and a set of overlying intersecting parallel electrodes, for instance of copper, which is oriented perpendicularly in relation to the underlying electrodes. The authors are aware of the sneak current problem when forming memory devices based on passive matrix addressing of this kind and hence in order to avoid erroneous read-out propose to add a layer of material between the Cu(TCNQ) film and the underlying electrode for forming a schottky barrier. Thus the problem with sneak currents is substantially eliminated and the use of M(TCNQ) in combination with the schottky barrier will hence be able to realize addressing of memory elements in large passive matrices. It is thus realized that in order to avoid the sneak current problem by addressing in large passive matrices of memory elements for storing of data, it is necessary to take the materials engineering conditions in regard. This is particularly important when one in addition to a pure memory function wishes to realize switching, registration or detecting functions in the matrix and wherein the current and voltage values may vary widely, such that a diode function is not always a necessary condition. It may also be the desired to combine the electrical addressing in a passive matrix with light emitting or light detecting devices, which make further demands on the material used, particularly when it is desired to realize passive matrices with for instance 10.sup.8 elements or cells per cm.sup.2.
Generally it has turned out difficult to address bistable or multistable switchable memory media in passive matrices and the problems with both addressability and reliable detection only increases with increasing numbers of nodes in the matrix, such as comprehensive simulation tests conducted by the applicant have shown. The same tests have also established that these problems may be surmounted by use of suitable materials with special electric or electronic properties.